88 modern sewer design

88 MODERN SEWER DESIGN

Fabricated fittings reduce head losses in the system.

894. HYDRAULICS OF STORM SEWERS

CHAPTER 4

Hydraulics ofStorm Sewers

Storm sewers may be designed as either open channels, where there is afree water surface, or for pressure or “pipe” flow under surcharged condi-tions. When the storm sewer system is to be designed as pressure flow itshould be assured that the hydraulic grade line does not exceed the floorlevel of any adjacent basements where surcharge conditions may createunacceptable flooding or structural damages.

Regardless of whether the sewer system is to be designed as an openchannel or pressure system, a thorough hydraulic analysis should be per-formed to assure that the system operates efficiently. Too often in the pasta simplistic approach to the design of storm sewers was taken, with thedesign and the sizing of the conduits and appurtenances derived fromnomographs or basic hydraulic flow equations.

As a result of this, excessive surcharging has been experienced in manyinstances due to improper design of the hydraulic structures. This in turnhas led to flooding damage, both surface and structural, when service con-nections have been made to the storm sewer. Overloading of the sewersystem may occur in upper reaches while lower segments may be flowingwell below capacity because of the inability of the upper reaches to trans-port the flow or vice versa with downstream surcharging creating prob-lems.

In conclusion, an efficient, cost effective storm system cannot be de-signed without a complete and proper hydraulic analysis.

The following section outlines the basic hydraulic principles for openchannel and conduit flow. Subsequent sections of this chapter deal withlosses (friction and form) within the sewer system and the hydraulics ofstorm water inlets. Manual calculations for designing a storm drainage sys-tem are presented in Chapter 5. An overview of several commonly usedcomputer programs which may be used to design sewer systems is alsogiven in Chapter 5.

CSP is easy to install in difficult trench conditions.


CLASSIFICATION OF CHANNEL FLOWChannel flow is distinguished from closed-conduit or pipe flow by the factthat the cross-section of flow is not dependent solely on the geometry ofthe conduit, but depends also on the free surface (or depth) which varieswith respect to space and time and is a function of discharge. As a result,various categories of flow can be identified:

STEADY flow exhibits characteristics at a point which is constant withrespect to time. Flow subject to very slow change may be assumed to besteady with little error.

UNSTEADY flow results when some time-dependent boundary condi-tion—tide, floodwave or gate movement causes a change in flow and/ordepth to be propagated through the system.

UNIFORM flow, strictly speaking, is flow in which velocity is the samein magnitude and direction at every point in the conduit. Less rigidly, uni-form flow is assumed to occur when the velocity at corresponding pointsin the cross-section is the same along the length of the channel. Note thatuniform flow is possible only if:—flow is steady, or nearly so—the channel is prismatic (i.e., has the same cross-sectional shape at all

sections)—depth is constant along the length of the channel—the bedslope is equal to the energy gradient.

NON-UNIFORM or VARIED flow occurs when any of the requirementsfor uniform flow are not satisfied. Varied flow may be further sub-classi-fied depending on the abruptness of the variation.

GRADUALLY VARlED flow occurs when depth changes occur overlong distances such as the flow profiles or backwater profiles which occurbetween distinct reaches of uniform flow.

RAPIDLY VARIED flow occurs in the vicinity of transitions caused byrelatively abrupt changes in channel geometry or where a hydraulic jumpoccurs.

Figure 4.1 illustrates various typical occurrences of these different classesof flow. In the design of sewer systems the flow, except where backwateror surcharging may occur, is generally assumed to be steady and uniform.

Laws of ConservationFluid mechanics is based on the law of conservation applied to the mass,energy and momentum of a fluid in motion. Full details can be found inany text on the subject. At this point, it is sufficient to note that:

1 ) Conservation of mass reduces to a simple statement of continuity forfluids in which the density is essentially constant.

2) Conservation of energy is usually stated as the Bernoulli equation whichis discussed below.

3) Conservation of momentum is significant in transitions where there arelocal and significant losses of energy, such as across a hydraulic jump.


Figure 4.1 Different classes of open channel flow

Bernoulli EquationThe law of conservation of energy as expressed by the Bernoulli Equationis the basic principle most often used in hydraulics. This equation may beapplied to any conduit with a constant discharge. All the friction flow for-mulae such as the Manning’s, Cutter, Hazer-William’s, etc., have been de-veloped to express the rate of energy dissipation as it applies to the BernoulliEquation. The theorem states that the energy head at any cross-section mustequal that in any other downstream section plus the intervening losses.1

In open channels the flow is primarily controlled by the gravitationalaction on moving fluid, which overcomes the hydraulic energy losses. TheBernoulli Equation defines the hydraulic principles involved in open chan-nel flow.

V1V2

Uniform FlowV1= V2

Gradually Varied Flow Gradually Varied Flow

Rapidly Varied FlowRapidly Varied Flow

(Hydraulic jump)

X


y1 V1

V1 2/2g

Z1

#1

Z2

V2 2/2g

EGLHGL

11

So

1

DATUM LINE

#2

hf

y2 V2

Figure 4.2 Energy in open channel flow

H = Total Velocity Head y = Water Depth

= Velocity Head

EGL = Energy Grade Line So = Slope of Bottom

V2

2g

hf = Headloss V = Mean Velocity Z = Height above DatumHGL = Hydraulic Grade Line Sf = Slope of EGL Sw = Slope of HGL

H = y + + Z + hfV2

2g

The total energy at point #1 is equal to the total energy at point #2 thus

yl + Zl + = y2 + Z2 + +hfV1

2

2gV2

2

2g

For pressure or closed conduit flow, the Bernoulli Equation can be written as:

+ + Zl = + + Z2 + hfV2

2

2gV1

2

2gP1

Where P = pressure at given location = specific weight of fluid

P2

V2

2/2g

#2

1

1

V1

V2

hf

Z1

#1

HorizontalLine

DATUM LINE

P1

EGL

HGL

Z2

P2

V1 2/2g

Figure 4.3 Energy in closed conduit flow

Sf

Sw

Sf

Sw


SPECIFIC ENERGYAn understanding of open channel flow is aided by the concept of SpecificEnergy E, which is simply the total energy when the channel bottom istaken to be the datum. Thus:

E = y + V2/2g = y + Q2/2gA2

Figure 4.4 shows a plot of specific energy as a function of depth of flowfor a known cross-sectional shape and constant discharge Q. The turningvalue occurs where E is a minimum and defines the critical depth ycr. Thecritical depth is defined by setting dE/dy = O from which it can be shownthat:

Q2 TgA3

in which the surface breadth, T and cross-sectional area, A are functions ofthe depth, y. The velocity corresponding to ycr is called the critical velocityand is given by:

V2cr TgA

or Vcr = (g A/T)1/2

Figure 4.4 Typical plot of specific energy as a function of depth

For the special case of rectangular cross-sections, A = B.y and T = B,where B is the basewidth. In this case the above equation for critical depthreduces to:

from which the critical depth is found as ycr = (Q2/gB2) 1/3 and the corre-sponding critical velocity is Vcr = (g.y)l/2.

=1 Q2

g.B3.y2

Subcritical range(upper-stage flow)

yc (critical depth)

yu

E

yLQ = constant

Specific energy head, E = y + = y +V2

2 g 2 g A2Q2

Supercritical range(lower-stage flow)

Dep

th, y

2 g

Vc2

2 g

VL2

2 g

Vu2

= 1

= 1


Figure 4.5 Critical flow and critical velocity in circular conduits

The critical depth serves to distinguish two more classes of openchannel flow:y > ycr The specific energy is predominantly potential energy (y), the

kinetic energy is small and the velocity is less than Vcr. The flowis called SUBCRITICAL (i.e., with respect to velocity) orTRANQUIL.

y < ycr Most of the specific energy is kinetic energy and the depth orpotential energy is small. The velocity is greater than Vcr and theflow is therefore called SUPERCRITICAL or RAPID.

For circular conduits Figure 4.5 provides a nomograph for calculatingycr.

For pipe arch CSP, pipe charts provide a graphical method of determin-ing critical flow depths (Figures 4.6, 4.7).

Q

D

D

1000

100

10

1

0.1

0.01

Discharge Q

(m3/s)

10000

1000

100

Diam

eter (mm

) Piv

ot L

ine

0.1

0.3

0.50.70.9

0.99

1

2

46

1040

Example : Given: Q = 0.03 m3/s D = 400 mm

Solution: Enter Q scale atQ = 0.03 m3/s Follow brokenline and turn at the Pivot Line.

ycr / D = 0.3 ycr = 120 mm

Vcr2 / D = 2.1 m; Vcr = 0.9 m/s

ycr

ycr / D (m)Vcr

2 / D (m)


ENERGY LOSSESWhen using the Bernoulli Equation for hydraulic design it is necessary tomake allowance for energy losses as illustrated in Figure 4.2. The lossesare expressed in terms of head and may be classified as:friction losses—these are due to the shear stress between the moving fluidand the boundary material.form losses—these are caused by abrupt transitions resulting from the ge-ometry of manholes, bends, expansions and contractions.

It is a common mistake to include only friction losses in the hydraulicanalysis. Form losses can constitute a major portion of the total head lossand, although estimates of form losses are generally based on empiricalequations, it is important to make allowance for them in the design.

Proper installation techniques are always important.


Figure 4.6A

Figure 4.6B

CRITICAL DEPTHSTANDARD C.S. PIPE-ARCH

Bureau of Public RoadsJan. 1964Federal Highway Administration2

Figure 4.6 Critical depth curves for standard corrugated steel pipe

700

600

500

400

300

200

100

00 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

ycr Cannot Exceed Top of Pipe

Pipe Arch

1030 mm x 740 mm 910 mm x 660 mm 680 mm x 500 mm 560 mm x 420 mm

Discharge - Q (m3/s)

Cri

tica

l Dep

th -

ycr

- (

mm

)

1200

1000

800

600

400

200

00.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.80


Pipe Arch



Cri

tica

l Dep

th -

ycr

- (

mm

)


Figure 4.7B

Figure 4.7A

Bureau of Public RoadsJan. 1964Federal Highway Administration2

Figure 4.7 Critical depth curves for structural plate pipe-arch

CRITICAL DEPTHSTRUCTURAL PLATEC.S. PIPE-ARCH

2000

1500

1000

500

00 1 2 3 4 5 6 7 8 9

Pipe Arch

Cri

tica

l Dep

th -

ycr

- (

mm

)




3500

3000

2500

2000

1500

1000

500

0

Cri

tica

l Dep

th -

ycr

- (

mm

)

0 5 10 15 20 25 30 35 40

Discharge - (m3/s)

Pipe Arch




Table 4.1 Waterway areas for standard sizes of corrugated steel conduits

Pipe-Arch StructuralRound Pipe (25mm Corrugation) Plate Arch

Diameter Area Size Area Size Area(mm) (m2) (mm) (m2) (mm) (m2)

300 0.07 1330 x 1030 1.09 1520 x 8100 0.98400 0.13 1550 x 1200 1.48 1830 x 8400 1.16500 0.2 1780 x 1360 1.93 1830 x 9700 1.39600 0.28 2010 x 1530 2.44 2130 x 8600 1.39700 0.39 2130 x 1120 1.86800 0.5 2440 x 1020 1.86900 0.64 2440 x 1270 2.42

1000 0.79 2740 x 1180 2.461200 1.13 2740 x 1440 3.071400 1.54 3050 x 1350 3.161600 2.01 3050 x 1600 3.811800 2.54 3350 x 1360 3.442000 3.14 3350 x 1750 4.65

3660 x 1520 4.183660 x 1910 5.483960 x 1680 5.023960 x 2060 6.54270 x 1840 5.954270 x 2210 7.434570 x 1870 6.414570 x 2360 8.554880 x 2030 7.434880 x 2520 9.755180 x 2180 8.555180 x 2690 11.065490 x 2210 9.015490 x 2720 11.715790 x 2360 10.225790 x 2880 13.016100 x 2530 11.526100 x 3050 14.59

Pipe-Arch(13 mm Corrugation)

Size(mm)

450 x 340

560 x 420

680 x 500

800 x 580

910 x 660

1030 x 740

1150 x 820

1390 x 970

1630 x 1120

1880 x 1260

2130 x 1400

Area(m2)

0.11

0.19

0.27

0.37

0.48

0.61

0.74

1.06

1.44

1.87

2.36

Structural Plate Pipe-Arch

Size(mm)

2060 x 15202240 x 16302440 x 17502590 x 18802690 x 20803100 x 19803400 x 20103730 x 22903890 x 26904370 x 28704720 x 30705050 x 33305490 x 35305890 x 37106250 x 39107040 x 40607620 x 4240

Area(m2)

2.492.93.363.874.494.835.286.618.299.7611.3813.2415.1

17.0719.1822.4825.27


T

D

B

= D

epth

of f

low

= R

ise

of c

ondu

it=

Spa

n of

con

duit

= A

rea

of fl

ow=

Hyd

raul

ic r

adiu

s=

Top

wid

th o

f flo

w

y D B A R T

y

Tabl

e 4.

4Va

lues

of

Det

erm

inat

ion

of to

p w

idth

.00

.01

.02

.03

.04

.05

.06

.07

.08

.09

.1.9

00.9

14.9

27.9

38.9

48.9

56.9

64.9

71.9

76.2

.982

.986

.990

.993

.995

.997

.998

.998

.998

.999

.3.9

97.9

96.9

95.9

93.9

91.9

89.9

87.9

85.9

82.9

79.4

.976

.971

.967

.964

.960

.956

.951

.947

.942

.937

.5.9

32.9

27.9

21.9

16.9

10.9

04.8

97.8

91.8

84.8

77.6

.870

.863

.855

.847

.839

.830

.822

.813

.803

.794

.7.7

84.7

73.7

63.7

52.7

41.7

29.7

17.7

04.6

91.6

78.8

.664

.649

.634

.618

.602

.585

.567

.548

.528

.508

.9.4

86.4

62.4

37.4

10.3

81.3

49.3

13.2

72.2

23.1

58

HY

DR

AU

LIC

PR

OP

ER

TIE

S O

F P

IPE

AR

CH

CO

ND

UIT

S F

LO

WIN

G P

AR

T F

UL

L

A BD

y D y Dy D

T BR D

Tabl

e 4.

2Va

lues

of

Det

erm

inat

ion

of a

rea

.00

.01

.02

.03

.04

.05

.06

.07

.08

.09

.1.0

72.0

81.0

90.1

00.1

09.1

19.1

28.1

38.1

48.2

.157

.167

.177

.187

.197

.207

.217

.227

.237

.247

.3.2

57.2

67.2

77.2

87.2

97.3

07.3

16.3

26.3

36.3

46.4

.356

.365

.375

.385

.394

.404

.413

.423

.432

.442

.5.4

51.4

60.4

70.4

79.4

88.4

97.5

06.5

15.5

24.5

33.6

.541

.550

.559

.567

.576

.584

.592

.600

.608

.616

.7.6

24.6

32.6

40.6

47.6

55.6

62.6

70.6

77.6

84.6

90.8

.697

.704

.710

.716

.722

.728

.734

.740

.745

.750

.9.7

55.7

60.7

64.7

69.7

72.7

76.7

80.7

83.7

85.7

871.

0.7

88

Tabl

e 4.

3Va

lues

of

Det

erm

inat

ion

of h

ydra

ulic

rad

ius

.00

.01

.02

.03

.04

.05

.06

.07

.08

.09

.1.0

78.0

86.0

94.1

02.1

10.1

18.1

26.1

33.1

41.2

.148

.156

.163

.170

.177

.184

.191

.197

.204

.210

.3.2

16.2

22.2

28.2

34.2

40.2

45.2

50.2

56.2

61.2

66.4

.271

.275

.280

.284

.289

.293

.297

.301

.305

.308

.5.3

12.3

15.3

19.3

22.3

25.3

28.3

31.3

34.3

37.3

39.6

.342

.344

.346

.348

.350

.352

.354

.355

.357

.358

.7.3

60.3

61.3

62.3

63.3

63.3

64.3

64.3

65.3

65.3

65.8

.365

.365

.364

.364

.363

.362

.361

.360

.359

.357

.9.3

55.3

53.3

50.3

48.3

44.3

41.3

37.3

32.3

26.3

181.

0.2

99


Tabl

e 4.

7Va

lues

of

Det

erm

inat

ion

of to

p w

idth

.00

.01

.02

.03

.04

.05

.06

.07

.08

.09

.0.0

00.1

99.2

80.3

41.3

92.4

36.4

57.5

10.5

43.5

72.1

.600

.626

.650

.673

.694

.714

.733

.751

.768

.785

.2.8

00.8

15.8

28.8

42.8

54.8

66.8

77.8

88.8

98.9

08.3

.917

.925

.933

.940

.947

.954

.960

.966

.971

.975

.4.9

80.9

84.9

87.9

90.9

93.9

95.9

97.9

98.9

991.

000

.51.

000

1.00

0.9

99.9

98.9

97.9

95.9

93.9

90.9

87.9

84.6

.980

.975

.971

.966

.960

.954

.947

.940

.933

.925

.7.9

17.9

08.8

98.8

88.8

77.8

66.8

54.8

42.8

28.8

15.8

.800

.785

.768

.751

.733

.714

.694

.673

.650

.626

.9.6

00.5

72.5

43.5

10.4

75.4

36.3

92.3

41.2

80.1

991.

0.0

00

Tabl

e 4.

6Va

lues

of

Det

erm

inat

ion

of h

ydra

ulic

rad

ius

.00

.01

.02

.03

.04

.05

.06

.07

.08

.09

.0.0

00.0

07.0

13.0

20.0

26.0

33.0

39.0

45.0

51.0

57.1

.063

.070

.075

.081

.087

.093

.099

.104

.110

.115

.2.1

21.1

26.1

31.1

36.1

42.1

47.1

52.1

57.1

61.1

66.3

.171

.176

.180

.185

.189

.193

.198

.202

.206

.210

.4.2

14.2

18.2

22.2

26.2

29.2

33.2

36.2

40.2

43.2

47.5

.250

.253

.256

.259

.262

.265

.268

.270

.273

.275

.6.2

78.2

80.2

82.2

84.2

86.2

88.2

90.2

92.2

93.2

95.7

.296

.298

.299

.300

.301

.302

.302

.303

.304

.304

.8.3

04.3

04.3

04.3

04.3

04.3

03.3

03.3

02.3

01.2

99.9

.298

.296

.294

.292

.289

.286

.283

.279

.274

.267

1.0

.250

Tabl

e 4.

5Va

lues

of

Det

erm

inat

ion

of a

rea

.00

.01

.02

.03

.04

.05

.06

.07

.08

.09

.0.0

00.0

01.0

04.0

07.0

11.0

15.0

19.0

24.0

29.0

35.1

.041

.047

.053

.060

.067

.074

.081

.089

.096

.104

.2.1

12.1

20.1

28.1

36.1

45.1

54.1

62.1

71.1

80.1

89.3

.198

.207

.217

.226

.236

.245

.255

.264

.274

.284

.4.2

93.3

03.3

13.3

23.3

33.3

43.3

53.3

63.3

73.3

83.5

.393

.403

.413

.423

.433

.443

.453

.462

.472

.482

.6.4

92.5

02.5

12.5

21.5

31.5

40.5

50.5

59.5

69.5

78.7

.587

.596

.605

.614

.623

.632

.640

.649

.657

.666

.8.6

74.6

81.6

89.6

97.7

04.7

12.7

19.7

25.7

32.7

38.9

.745

.750

.756

.761

.766

.771

.775

.779

.782

.784

1.0

.785

y D y D

A D 2

R D

T

Y

D

y D

T D

Fro

m ta

bles

;

=

0.6

32,

=

0.30

2,

=

0.8

66R D

A D 2

T D

D =

Dia

met

ery

= D

epth

of f

low

A =

Are

a of

flo

wR

= H

ydra

ulic

rad

ius

T =

Top

wid

th

HY

DR

AU

LIC

PR

OP

ER

TIE

S O

F C

IRC

ULA

RC

ON

DU

ITS

FLO

WIN

G P

AR

T FU

LL

i.e. G

iven

y =

300

mm

, D =

400

mm

,

= 0.

75y D


WhereV = average velocityQ = dischargeR = hydraulic radius = A/PA = cross-sectional areaP = wetted perimeterSf = friction gradient or slope of energy linen = Manning’s roughness coefficient (see Tables 4.8, 4.9, 4.10)

FRICTION LOSSESIn North America, the Manning and Kutter equations are commonly usedto estimate the friction gradient for turbulent flow in storm sewers. In bothequations fully developed rough turbulent flow is assumed so that the headloss per unit length of conduit is approximately proportional to the squareof the discharge (or velocity). Both equations use an empirical coefficient‘n’ to describe the roughness of the channel boundary. Tables 4.9 and 4.10give suggested values for ‘n’ for various corrugation profiles and linings.

Manning EquationThe Manning Equation is one of a number of so-called exponential equa-tions. It is widely used in open channel flow but can also be applied toclosed conduit flow. The equation is not dimensionally homogeneous anda correction factor must be applied depending upon the system of unitsbeing used.

V = R2/3 Sf 1/21n

(m/s)(m3/s)(m)(m2)(m)

Table 4.8 Effective absolute roughness and friction formula coefficients3

Conduit Material Manning n

Closed conduitsAsbestos-cement pipe 0.011-0.015Brick 0.013-0.017Cast iron pipe

Uncoated (new) –Asphalt dipped (new) –Cement-lined & seal coated 0.011-0.015

Concrete (monolithic)Smooth forms 0.012-0.014Rough forms 0.015-0.017

Concrete pipe 0.011-0.015Plastic pipe (smooth) 0.011-0.015Vitrified clay

Pipes 0.011-0.015Liner plates 0.013-0.017

Open channels Lined channels

a. Asphalt 0.013-0.017b. Brick 0.012-0.018c. Concrete 0.011-0.020d. Rubble or riprap 0.020-0.035e. Vegetal 0.030-0.400

Excavated or dredgedEarth, straight and uniform 0.020-0.030Earth, winding, fairly uniform 0.025-0.040Rock 0.030-0.045Unmaintained 0.050-0.140

Natural Channels (minor streams, top width at flood stage <30m)Fairly regular section 0.030-0.0700Irregular section with pools 0.040-0.100


C =1 + n

R( )0.001 55S

23 +

0.001 55S

1n23 + +

Figure 4.8 provides nomographs for estimating steady uniform flows forpipes flowing full, using the Manning equation. In cases where conduitsare flowing only partly full, the corresponding hydraulic ratios may bedetermined from Figures 4.9 and 4.10.

Kutter EquationThe Kutter Equation is used for open channel calculations in certain areasof the United States. It is an empirically derived relation between the Chezycoefficient ‘C’ and the Manning roughness coefficient ‘n.’

Q = A.C.R1/2 . Sf

1/2

where

℘

Table 4.9 Values of coefficient of roughness (n) for standard corrugatedsteel pipe (Manning’s formula)*

Helical

68 x 13mm

1400 &Corrugations 200 250 300 400 500 600 900 1200 Larger

Unpaved 0.024 0.012 0.011 0.013 0.014 0.015 0.018 0.018 0.020 0.02125% Paved 0.021 0.014 0.017 0.020 0.019Fully Paved 0.012 0.012 0.012 0.012 0.012

Helical - 76 x 25mm

2200 &1200 1400 1600 1800 2000 Larger

Unpaved 0.027 0.023 0.023 0.024 0.025 0.026 0.02725% Paved 0.023 0.020 0.020 0.021 0.022 0.022 0.023Fully Paved 0.012 0.012 0.012 0.012 0.012 0.012 0.012

Helical - 125 x 26mm

2000 &1400 1600 1800 Larger

Unpaved 0.025 0.022 0.023 0.024 0.02525% Paved 0.022 0.019 0.020 0.021 0.022Fully Paved 0.012 0.012 0.012 0.012 0.012

*AISI

Table 4.10 Values of n for structural plate pipe for152 x 51mm corrugations (Manning’s formula)

DiametersCorrugations152 x 51mm 1500mm 2120mm 3050mm 4610mm

Plain – unpaved 0.033 0.032 0.030 0.02825% Paved 0.028 0.027 0.026 0.024

Annular68 x 13 mm

AllDiametres

Annular76 x 25 mm

Annular125 x 26 mm

38 x 65 mm


Although the friction slope Sf appears as a second order term in the ex-pression for ‘C’ the resulting discharge is not sensitive to this term. Table4.11 shows the difference (%) in discharge computed using the Kutter equa-tion compared with that obtained by Manning. The table gives the relation-ship between the diameter (D) and the hydraulic radius (R) assuming fullflow in a circular pipe. The values in Table 4.11 are also valid for noncircularpipes flowing partially full.

70,00060,000

50,000

40,000

30,000

20,000

10,00090008000700060005000

4000

3000

2000

1000900800700600500

400

300

200

10090807060

50

40

30

20

10

4000

3000

2000

1000900800

700

600

500

400

300

200

100

0.03

0.04

0.05

0.06

0.070.08

0.090.1

0.2

0.3

0.4

0.5

0.6

0.7

0.80.9

DIS

CH

AR

GE

IN L

ITR

ES

PE

R S

EC

ON

D, Q

DIA

ME

TE

R O

F P

IPE

, IN

MIL

LIM

ET

RE

S, D

HY

DR

AU

LIC

RA

DIU

S, I

N M

ET

RE

S, R

TU

RN

ING

LIN

E

VE

LOC

ITY

, IN

ME

TR

ES

PE

R S

EC

ON

D, V

SLO

PE

, S

MA

NN

ING

’S R

OU

GH

NE

SS

CO

EF

FE

CIE

NT

, n 0.080.10

0.060.050.04

0.03

0.02

0.015

0.0100.008

0.006

0.2

0.4

0.3

0.2

0.1

0.08

0.060.050.04

0.03

0.02

0.01

0.008

0.0060.0050.004

0.003

0.002

0.0010.0008

0.00060.00050.0004

0.0003

0.0002

0.00010.00008

0.000060.000050.00004

0.00003

0.00002

0.00001

0.000008

0.0000060.0000050.000004

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

2

3

4

5

Q D/R n V S

Alignment chart for energy loss in pipes, for Manning’s formula.Note: Use chart for flow computations, H

L = S

Figure 4.8 Nomograph for solution of Manning’s formula


FULL

90

80

70

60

50

40

30

20

10

0.1 .2 .3 .4 .5 .6 .7 .8 .9 1.0 1.1 1.2 1.3

Wetted perimeter

Area

Discharge

Hydraulic radius

Proportional values based on full conditions

Figure 4.9 Hydraulic properties of corrugated steel and structural plate pipe-arches

Per

cent

of t

otal

ris

e

FULL

90

80

70

60

50

40

30

20

10

0.1 .2 .3 .4 .5 .6 .7 .8 .9 1.0 1.1 1.2 1.3

Velocity

Area

Discharge Hydraulic radius

Hydraulic elements in terms of hydraulic for full section

Figure 4.10 Hydraulic elements graph for circular CSP

Rat

io o

f dep

th o

f flo

w to

dia

met

er y

/D


Table 4.11 Percent difference of Kutter equation comparedwith Manning equation (Grade = 1.0%)

n 0.013 0.02 0.03

D-metres R-metres

0.5 0.125 -0.31 -5.50 -6.741 0.25 1.15 -2.20 -3.62

1.5 0.375 1.34 -0.96 -2.192 0.5 1.20 -0.38 -1.35

2.5 0.625 0.94 -0.11 -0.823 0.75 0.64 0.01 -0.45

3.5 0.875 0.32 0.03 -0.194 1 0.00 0.00 0.00

4.5 1.125 -0.32 -0.07 0.145 1.25 -0.62 -0.16 0.24

5.5 1.375 -0.92 -0.27 0.316 1.5 -1.21 -0.39 0.36

The two equations give identical results for values of R close to l.0mwhich represents a very large pipe of perhaps 3600mm diameter. For smallersized conduits, the difference is significant especially where the roughnesscoefficient is large.

SOLVING THE FRICITION LOSS EQUATIONOf the three quantities (Q, Sf, yo) of greatest interest in open channel analy-sis the discharge Q and the friction slope Sf are easily obtained as theyappear explicitly in the equations. Because of the exponential form of theManning equation it is a simple matter to compute the friction slope Sf as afunction of velocity or discharge for known cross-sectional properties. Evenwith the Kutter equation, the second order term in Sf is of little importanceand can be safely ignored as a first iteration when solving for Sf.

The third quantity is the normal depth yo, which is the depth at whichuniform flow would take place in a very long reach of channel. The normaldepth is less easily determined as it appears in the expressions for botharea A and hydraulic radius R. A trial and error solution is required exceptfor sections of straightforward geometry.

For partially-full circular channels a convenient semi-graphical methodof solution is provided by the curves describing proportional ratios of dis-charge, hydraulic radius, area and velocity expressed as a function of therelative depth y/D. Two simple examples should give an indication of howthese curves can be used:Example 1: Finding the normal depth yo.A pipe of diameter l.0m (n = 0.013) has a gradient of 1.0% . It is requiredto find the normal depth yo for a discharge of 2 m3/sStep 1: Calculate the full-pipe capacity using Manning’s equation forD = l.0 mFor full-pipe flow R = D/4 = 0.25 mQ = (1)2 (0.25)2/3 (0.01) 1/2

/ 0.0l3 = 2.4 m 3/s≠4


Step 2: Get the proportional discharge Qact/Qfull = 2/2.4 = 0.83Step 3: From the ‘Discharge’ curve of Figure 4.10 find the corresponding

proportional depth y/D = 0.68. Thus the normal depth is given by:yo = 0.68x 1= 0.68m

Example 2:Designing for a range of flows.A pipe is designed to carry a minimum discharge of 0.12m3/s. With a veloc-

ity not less than 1.0m/s and a maximum discharge 0.6m3/s without surcharg-ing. Use the flattest gradient possible. (n = 0.013)Step 1: Assuming Qfull = Qmax = 0.6 ; Qmin / Qfull = 0.12 / 0.6 = 0.2Step 2: This corresponds to y/D = 0.31 which in turn corresponds to a

proportional velocity of Vmin / Vfull = 0.78 (Figure 4.9).Thus the full pipe velocity corresponding to Vmin = 1.0 m/s isgiven by:Vfull = 1.0 / 0.78 = 1.28 m/s

Step 3: Thus for full pipe flow the required section area is given by:A = Qmax / Vfull = 0.6 / 1.28 = 0.47m2

or D = (4 A/≠)1/2 = 0.77mStep 4: Assuming that commercial sizes are available in increments of

100mm the selected diameter must be rounded down (to ensure Vmin

> 1.0 m/s) to 700mmStep 5: The necessary slope is then obtained from the Manning equation as

where A = ≠ D 2/4 = 0.38m2 and R = D/4 = 0.175mThus the required grade is So = 0.0043 or approximately 0.4%

SURFACE WATER PROFILESUniform flow is seldom attained except in very long reaches, free from anyform of transition. Gradually varied flow occurs as a form of gentle transitionfrom one stage of uniform flow to another and non-uniform flow is found to bethe rule rather than the exception.

The flow profiles of gradually varied flow can be classified in relation to thenormal depth yo and the critical depth ycr and the slope of the channel.Channel slope is described as:

(I) MILD when yo > ycr i.e. So < Scr.(2) STEEP when yo < ycr i.e. So > Scr.

Note that the critical slope Scr is slightly dependent on the stage or magni-tude of flow, so that strictly speaking the description of Mild or Steep shouldnot be applied to the channel without regard to the flow conditions.

Most textbooks show five classes of channel slope: Mild, Steep, Critical,Horizontal and Adverse. In practice the last three categories are special casesof the first two and it is sufficient to consider them. In addition to the channelslope, a profile of gradually varied flow can be classified depending on whetherit lies above, below or between the normal and critical depths. The three zonesmay be defined as follows.

Zone I — Profile lies above both yo and ycr

Zone 2 — Profile lies between yo and ycr

Zone 3 — Profile lies below both yo and ycr

So = Sr =A2 R4/3

Q2 n2


Using the capitals ‘M’ and ‘S’ to denote Mild or Steep channel state andthe Zone number ‘1’, ‘2’ or ‘3,’ profiles may be classified as ‘M1’ or ‘S3.’Figure 4.11 shows the idealized cases of the six basic profile types alongwith typical circumstances in which they can occur.

Horizontal

1

3

2

Horizontal

1

yn > ycr > y

yn > y > ycr

y > yn > ycr

M2

M1

M3ycr

ynycr

yn

S1

S2

S3

Mild slopeso > 0.0, yn > ycr

Steep slopeso > 0.0, yn < ycr

M1

ycr

ycr

yn

S1

M2

ynycr

yn

M1

M3 ycryn

yn

ycr

ycr

yn

S2

S3

Figure 4.11 Idealized flow profile

3

2


HYDRAULIC JUMPWhen supercritical flow enters a reach in which the flow is subcritical, anabrupt transition is formed which takes the form of a surface roller or undularwave which tries to move upstream but which is held in check by the ve-locity of the supercritical flow. Figure 4.12 shows a typical situation inwhich supercritical uniform flow from a steep reach enters a reach of mildslope in which the normal depth is subcritical.

The energy losses associated with the violent turbulence of the hydrau-lic jump make application of the Bernoulli equation impossible. Instead,the control volume of fluid containing the jump can be analyzed using theequation of conservation of momentum. For a prismatic channel of arbi-trary cross-section this can be expressed as follows:

Q2/(g A1) + A1 y1 = Q2/(g A2) + A2y2

where y = depth to the centroid of the cross-section A = cross-sectional area

Q = total discharge g = gravitational acceleration

For the special case of a rectangular cross-section, the solution can beobtained directly using the discharge per unit breadth:

y2 = –(y1/2) + (y12/4 + 2q2/(gy1))1/2

where y2 = depth downstream of the jump y1 = depth upstream of the jump

q = discharge per unit breadth of channel g = gravitational acceleration

The above equation is reversible so that y1 may be found as a function ofy2 using a similar relationship.

FORM LOSSES IN JUNCTIONS, BENDS AND OTHER STRUCTURESFrom the time storm water first enters the sewer system at the inlet until itdischarges at the outlet, it will encounter a variety of hydraulic structuressuch as manholes, bends, contractions, enlargements and transitions, whichwill cause velocity head losses. These losses have been called “minorlosses”. This is misleading. In some situations these losses are asimportant as those arising from pipe friction. Velocity losses may be ex-pressed in a general form derived from the Bernoulli and Darcy-Weisbachequations.

ycr

y1

V1V2

y2

Figure 4.12 Hydraulic jump


where: H = velocity head lossK = coefficient for the particular structure

The following are useful velocity head loss formulae of hydraulic struc-tures commonly found in sewer systems. They are primarily based on ex-periments.

Transition Losses (open channel)The energy losses may be expressed in terms of the kinetic energy at thetwo ends:

where Kt is the transition loss coefficient

Contraction:

Expansion:

Where V1 = upstream velocityV2 = downstream velocity

Simple transition in size in a manhole with straight-through flow may beanalyzed with the above equations.

Transition Losses (pressure flow)Contraction:

K = 0.5 for sudden contractionK = 0.1 for well designed transition

and A1, A2 = cross-sectional area of flow of incoming and outgoing pipe from transition.

Expansion:

K = 1.0 for sudden expansionK = 0.2 for well designed transition

The above K values are for estimating purposes. If a more detailed analy-sis of the transition losses is required, then the tables in conjunction withthe energy losses equation in the form below should be used for pressureflow.

( ) [ ]( )

H = K V2

2g

Ht = Kt [ ]V2

2g

Ht = 0.1

Ht = 0.2

Ht = K

Ht = K[ ]( V1 — V2 )2

2g

Ht = K

when V2 > V1

when V1 > V2

V22

2g1

2A2

A1

( )V2

2g

( )V22

2gV1

2

2g

( )V12

2gV2

2

2g


Table 4.13 Values of K2 for determining loss of head due to gradualenlargement in pipes, from the formula H2 = K2(V1

2/2g)

d2/d1 = ratio of diameter of larger pipe to diameter of smaller pipe. Angle of cone istwice the angle between the axis of the cone and its side.

Angle of cone

2ϒ 4ϒ 6ϒ 8ϒ 10ϒ 15ϒ 20ϒ 25ϒ 30ϒ 35ϒ 40ϒ 45ϒ 50ϒ 60ϒ

1.1 .01 .01 .01 .02 .03 .05 .10 .13 .16 .18 .19 .20 .21 .231.2 .02 .02 .02 .03 .04 .09 .16 .21 .25 .29 .31 .33 .35 .371.4 .02 .03 .03 .04 .06 .12 .23 .30 .36 .41 .44 .47 .50 .531.6 .03 .03 .04 .05 .07 .14 .26 .35 .42 .47 .51 .54 .57 .611.8 .03 .04 .04 .05 .07 .15 .28 .37 .44 .50 .54 .58 .61 .652.0 .03 .04 .04 .05 .07 .16 .29 .38 .46 .52 .56 .60 .63 .682.5 .03 .04 .04 .05 .08 .16 .30 .39 .48 .54 .58 .62 .65 .703.0 .03 .04 .04 .05 .08 .16 .31 .40 .48 .55 .59 .63 .66 .71

� .03 .04 .05 .06 .08 .16 .31 .40 .49 .56 .60 .64 .67 .72

Table 4.12 Values of K2 for determining loss of head due to suddenenlargement in pipes, from the formula H2 = K2(V1

2/2g)

d2/d1 = ratio of larger pipe to smaller pipe V1 = velocity in smaller pipe

Velocity, V1, in metres per second

0.6 0.9 1.2 1.5 1.8 2.1 2.4 3.0 3.6 4.5 6.0 9.0 12.0

1.2 .11 .10 .10 .10 .10 .10 .10 .09 .09 .09 .09 .09 .08 1.4 .26 .26 .25 .24 .24 .24 .24 .23 .23 .22 .22 .21 .20 1.6 .40 .39 .38 .37 .37 .36 .36 .35 .35 .34 .33 .32 .32 1.8 .51 .49 .48 .47 .47 .46 .46 .45 .44 .43 .42 .41 .40 2.0 .60 .58 .56 .55 .55 .54 .53 .52 .52 .51 .50 .48 .47 2.5 .74 .72 .70 .69 .68 .67 .66 .65 .64 .63 .62 .60 .58 3.0 .83 .80 .78 .77 .76 .75 .74 .73 .72 .70 .69 .67 .65 4.0 .92 .89 .87 .85 .84 .83 .82 .80 .79 .78 .76 .74 .72 5.0 .96 .93 .91 .89 .88 .87 .86 .84 .83 .82 .80 .77 .7510.0 1.00 .99 .96 .95 .93 .92 .91 .89 .88 .86 .84 .82 .80 � 1.00 1.00 .98 .96 .95 .94 .93 .91 .90 .88 .86 .83 .81

d2

d1

d2

d1


Manhole LossesManhole losses in many cases comprise a significant percentage of theoverall losses within a sewer system. Consequently, if these losses are ig-nored, or underestimated, the sewer system may surcharge leading to base-ment flooding or sewer overflows. Losses at sewer junctions are depend-ent upon flow characteristics, junction geometry and relative sewer diam-eters. General problems with respect to flow through junctions have beendiscussed by Chow8 who concluded that the losses could be best estimatedby experimental analysis as opposed to analytical procedures.

Table 4.15 Entrance loss coefficients for corrugated steel pipe or pipe-arch

Inlet End of Culvert Coefficient Ke

Projecting from fill (no headwall) 0.9 Headwall, or headwall and wingwalls square-edged 0.5 Mitered (beveled) to conform to fill slope 0.7*End-Section conforming to fill slope 0.5 Headwall, rounded edge 0.2 Beveled Ring 0.25

*End Sections available from manufacturers.

Entrance Losses

He = KeV2

2g

d2

d1

Table 4.14 Values of K3 for determining loss of head due to suddencontraction from the formula H3 = K3(V2

2/2g)

d2/d1 = ratio of larger to smaller diameter V2 = velocity in smaller pipe

Velocity, V2, in metres per second

0.6 0.9 1.2 1.5 1.8 2.1 2.4 3.0 3.6 4.5 6.0 9.0 12.0

1.1 .03 .04 .04 .04 .04 .04 .04 .04 .04 .04 .05 .05 .06 1.2 .07 .07 .07 .07 .07 .07 .07 .08 .08 .08 .09 .10 .11 1.4 .17 .17 .17 .17 .17 .17 .17 .18 .18 .18 .18 .19 .20 1.6 .26 .26 .26 .26 .26 .26 .26 .26 .26 .25 .25 .25 .24 1.8 .34 .34 .34 .34 .34 .34 .33 .33 .32 .32 .31 .29 .27 2.0 .38 .38 .37 .37 .37 .37 .36 .36 .35 .34 .33 .31 .29 2.2 .40 .40 .40 .39 .39 .39 .39 .38 .37 .37 .35 .33 .30 2.5 .42 .42 .42 .41 .41 .41 .40 .40 .39 .38 .37 .34 .31 3.0 .44 .44 .44 .43 .43 .43 .42 .42 .41 .40 .39 .36 .33 4.0 .47 .46 .46 .46 .45 .45 .45 .44 .43 .42 .41 .37 .34 5.0 .48 .48 .47 .47 .47 .46 .46 .45 .45 .44 .42 .38 .35 10.0 .49 .48 .48 .48 .48 .47 .47 .46 .46 .45 .43 .40 .36 � .49 .49 .48 .48 .48 .47 .47 .47 .46 .45 .44 .41 .38


Marsalek9, in a study for three junction designs found the following:i) In pressurized flow the most important flow variable was the relative

lateral inflow for junctions with more than two pipes. The losses increased as the ratio of the lateral discharge to main line discharge

increased.ii) Among the junction geometrical parameters, the important ones are:

relative pipe sizes, junction benching and pipe alignment. Base shapeand relative manhole sizes were less influential.

iii) Full benching to the crown of the pipe significantly reduced losses ascompared to benching to the mid-section of the pipe or no benching.

iv) In junctions where two lateral inflows occurred, the head lossesincreased as the difference in flows between the two lateral sewersincreased. The head loss was minimized when the lateral flowswere equal.

Various experimental studies 10, 1l, 12, 13, 14, 15 have been performed to esti-mate manhole losses. These works should be referred to whenever possi-ble. In cases where no applicable results are available, the following maybe used as a guideline to estimate manhole losses.

Manhole Losses (flow straight through)In a straight through manhole where there is no change in pipe size, lossescan be estimated by:

Terminal Manhole LossesLosses at terminal manholes may be estimated by the formula:

Manhole Junction LossesLosses at junctions where one or more incoming laterals occur may beestimated by combining the laws of pressure plus momentum where Hj isequal to the junction losses.

Using the laws of pressure plus momentum:

Hm = 0.05 V2

2g

Htm = V2

2g

Hj = Kj V2

2g

1 2Flow

3

( Hj + D1 – D2) ( A1 + A2)

2=

Q22

A2g Q1

2

A1g–

Q32

A3gcos 0

0

–


Bend LossesBend losses may be estimated from the equation:

For curved sewer segments where the angle is less than 40ϒ the bend losscoefficient may be estimated as:

where: O = central angle of bend in degreesFor greater angles of deflection and bends in manholes the bend loss

coefficient may be determined from Figure 4.13.

HYDRAULICS OF STORM INLETS

Hydraulics of Storm Water InletsStorm water inlets are the means by which storm runoff enters the sewersystem. Their design is often neglected or receives very little attention dur-ing the design of storm drainage systems. Inlets play an important role inroad drainage and storm sewer design because of their effect on both therate of water removal from the road surface and the degree of utilization ofthe sewer system. If inlets are unable to discharge the design inflow to thesewer system it may result in a lower level of roadway convenience andconditions hazardous to traffic. It may also lead to overdesign of the sewerpipes downstream of the inlet. In some cases the limited capacity of theinlets may be desirable as a storm water management alternative therebyoffering a greater level of protection from excessive sewer surcharging. Insuch cases, both the quantity of runoff intercepted and the resulting levelof roadway convenience must be known. Furthermore, overdesign in thenumber of inlets results in higher costs and could result in over-utilizationof the sewer system.

No one inlet type is best suited for all conditions. Many different typesof inlets have thus been developed, as shown in Figure 4.17. In the past,the hydraulic capacities of some of these inlets was often unknown, some-times resulting in erroneous capacity estimates.

Storm water inlets may not intercept all runoff due to the velocity offlow over the inlet and the spread of flow across the roadway and gutter.This leads to the concept of carryover flow. As carryover flow progressesdownstream, it may accumulate, resulting in a greater demand for inter-ception. It is imperative that more emphasis be placed on inlet design toassure that the inlet type, location and capacity are adequately determinedto achieve the overall drainage requirements.

The hydraulic efficiency of inlets is a function of street grade, cross-slope, inlet geometry and curb and gutter design. Generally, an increasedstreet cross-slope will result in increased inlet capacity as the flow is con-centrated within the gutter. The depth of flow in the gutter may be esti-mated from Figure 4.14. The effect of street grades on inlet capacities

Hb = Kb V2

2g

Kb = 0.25 o90


Figure 4.13 Sewer bend loss coefficient16

0ϒ 20ϒ 40ϒ 60ϒ 80ϒ 90ϒ 100ϒ

0.0

1.4

1.2

1.0

0.8

0.6

0.4

0.2

Lo

ss C

oef

ficie

nt,

Kb

D

O

O Bend at Manhole,Curved or Deflector

Bend at Manhole,no Special Shaping

Deflector

Curved

Curved Sewer r / D = 2

Sewer r / D > 6

Deflection Angle O, Degrees

r


Compacting backfill is required for proper installation of all sewers.

varies. Initially as the street grade increases there is an increase in gutterflow velocity which allows a greater flow to reach the inlets for intercep-tion. However, as street grades continue to increase there is a thresholdwhere the velocity is so high that less flow can be intercepted. This thresh-old velocity depends upon the geometry of the inlet and characteristics ofthe gutter, see Figures 4.15 and 4.16.

Experiments on inlet capacities17 have resulted in a set of tables and chartsto aid the designer in storm water inlet selection and sewer system design.A sample of the results is shown in Figures 4.15 and 4.16, Tables 4.16 and4.17.

To use these charts or tables the designer determines the overland flowand the resulting spread in gutter flow from a pre-determined road gradeand crossfall, gutter design and inlet type; see Table 4.16. This value isthen used with Table 4.17 to obtain the storm water inlet or grate inletcapacity. The difference between the flow on the roadway and the inletcapacity is referred to as the carryover. An illustrative example is presentedbelow:Design Parameter — Road crossfall = 0.02 m/m

— Road grade = 0.02 m/m— Gutter type B— Inlet grate type = OPSD-400 (Figure 4.16)— One inlet on each side of the road— Upstream carryover flow = 0 m3/s

Catchment Runoff = 0.18 m3/sGutter Flow = 0.18 = 0.09 m3/s

2


Figure 4.14 Nomograph for flow in triangular channels

10000

8000

60005000

4000

30002000

1000

800

600500

400

300

200

10080

6050

40

30

20

10

10.0

3.02.01.0

.30

.20

.10

.03

.02

.01

.003

.002

.001

.0003

.06

.05

.04

.03

.02

.01

.008

.007

.006

.005

.004

.003

.002

.001

.10

.7

.6

.5

.4

.3

.2

.09

.1

.08

.07

.06

.05

.04

.03

.02

.01

.009

.008

.007

.006

.005

.004

.003

.08

.07

DIS

CH

AR

GE

(Q

) m

3 /s

SLO

PE

OF

CH

AN

NE

L (S

) IN

m/m

DE

PT

H A

T C

UR

B O

R D

EE

PE

ST

PO

INT

(d)

IN m

Reference H. R. B. proceedings 1946,page 150, equation (14)

Example (see dashed lines)Given: s = 0.03

z = 24n = .02d = .067 m

Find: Q = .056 m3/s

INSTRUCTIONS1. Connect z/n ratio with slope(s) and connect discharge (Q) with depth (d). These two lines must intersect at turning line for complete solution.

2. For shallow v-shaped channel as shown use nomograph

with z = T–d

x–z

3. To determine discharge Qx in portion of channel having width x: determine depth d for total discharge in entire section a. Then use nomograph to determine Qb in section b for depth

d1 = d – ( )

4. To determine discharge in composite section: follow instruction 3 to obtain discharge in section a at assumed depth d: obtain Qb for slope ratio Zb and depth d1, then QT = Qa • Qb

Z d

d

T

d

d

x

b

a d1

x–z( )

d

x zbd1

ba

d1

x–za

= za (d – d1)

Equation: Q = .56 (Z) s1/2 d8/3

n is roughness coefficient in Manningformula appropriate to material inbottom of channelZ is reciprocal of cross slope

n

z/n = 1200

TU

RN

ING

LIN

E

RA

TIO

Z/n


2.5 m

2.0

1.5

1.0

0.80.5

Sx = 0.04

0.08

0.06

0.04

0.02

0.000.00 0.05

Grade0.10m/m

Sx = 0.06

0.00 0.05Grade

0.10m/m

0.06

0.04

0.02

0.00

Sx = 0.06

0.08

0.06

0.04

0.02

0.000.00 0.05

Grade0.10m/m

1.5

1.0

0.8

0.5

Sx = 0.04

0.06

0.04

0.02

0.000.00 0.05

Grade0.10m/m

3.02.72.5

2.0

1.5

1.00.80.5

Figure 4.15 Sewer inlet capacity: as per curb and gutter in Figure 4.16

600 mm

150 mm 50 mm 400 mm

75 mm

50 mm

Crossfall = 0.02

Curb & GutterType B

Curb Side 48 mm

604 mm

552

mm

600

mm

Figure 4.16Catch basin grate

Sx = crossfall T = spread

INLE

T C

AP

AC

ITY

(m

3 /s)

INLE

T C

AP

AC

ITY

(m

3 /s)

1.0 m

0.8

0.5

T =

T =


Table 4.16 Gutter flow rate17 (m3/s)

Grade (m/m)Crossfall Spread Depth(m/m) (m) (m) 0.003 0.01 0.02 0.03 0.04 0.06 0.08 0.1

0.00 0.05 0.005 0.008 0.012 0.014 0.016 0.020 0.023 0.0260.50 0.06 0.008 0.014 0.020 0.024 0.028 0.034 0.039 0.0440.75 0.06 0.010 0.018 0.025 0.031 0.036 0.044 0.051 0.0571.00 0.07 0.013 0.024 0.033 0.041 0.047 0.058 0.067 0.074

0.02 1.50 0.08 0.022 0.039 0.055 0.068 0.078 0.096 0.110 0.1232.00 0.09 0.034 0.062 0.087 0.107 0.123 0.151 0.175 0.1952.50 0.10 0.051 0.093 0.131 0.161 0.186 0.227 0.263 0.2942.70 0.10 0.059 0.108 0.153 0.187 0.216 0.264 0.305 0.3413.00 0.11 0.073 0.134 0.189 0.231 0.267 0.327 0.378 0.4220.50 0.07 0.012 0.022 0.030 0.037 0.043 0.053 0.061 0.0680.75 0.08 0.018 0.033 0.046 0.057 0.066 0.080 0.093 0.104

0.04 1.00 0.09 0.026 0.048 0.068 0.084 0.097 0.118 0.136 0.1531.50 0.11 0.051 0.094 0.133 0.162 0.188 0.230 0.265 0.2962.00 0.13 0.089 0.163 0.230 0.281 0.325 0.398 0.460 0.5142.50 0.15 0.142 0.258 0.365 0.447 0.517 0.633 0.731 0.8170.50 0.08 0.017 0.031 0.043 0.053 0.061 0.075 0.087 0.0970.75 0.09 0.028 0.052 0.073 0.089 0.103 0.126 0.146 0.163

0.06 1.00 0.11 0.044 0.080 0.114 0.140 0.161 0.197 0.228 0.2551.50 0.14 0.092 0.168 0.237 0.290 0.335 0.411 0.474 0.5301.67 0.15 0.113 0.206 0.292 0.358 0.413 0.506 0.584 0.6530.50 0.09 0.023 0.042 0.059 0.072 0.083 0.102 0.117 0.131

0.08 0.75 0.11 0.040 0.074 0.104 0.128 0.148 0.181 0.209 0.2341.00 0.13 0.065 0.120 0.169 0.207 0.239 0.293 0.338 0.3781.25 0.15 0.099 0.181 0.255 0.313 0.361 0.442 0.511 0.571

Table 4.17 Grate inlet capacity17 (m3/s)*

Grade (m/m)Crossfall Spread(m/m) (m) 0.00 0.01 0.02 0.03 0.04 0.06 0.08 0.10

0.50 0.005 0.007 0.010 0.011 0.012 0.012 0.013 0.0120.75 0.008 0.012 0.014 0.017 0.018 0.019 0.019 0.0171.00 0.010 0.014 0.018 0.021 0.022 0.023 0.024 0.022

0.02 1.50 0.013 0.023 0.029 0.031 0.033 0.035 0.034 0.0322.00 0.023 0.035 0.040 0.043 0.044 0.044 0.043 0.0412.50 0.034 0.046 0.052 0.054 0.054 0.054 0.052 0.0502.70 0.037 0.050 0.056 0.057 0.058 0.057 0.056 0.0523.00 0.042 0.055 0.061 0.062 0.062 0.061 0.059 0.0570.50 0.007 0.013 0.017 0.020 0.022 0.024 0.024 0.0210.75 0.012 0.021 0.027 0.030 0.031 0.032 0.031 0.028

0.04 1.00 0.016 0.027 0.035 0.039 0.040 0.042 0.040 0.0381.50 0.027 0.046 0.054 0.057 0.058 0.056 0.053 0.0502.00 0.042 0.064 0.070 0.071 0.071 0.070 0.068 0.0642.50 0.057 0.078 0.081 0.081 0.080 0.076 0.073 0.0720.50 0.010 0.015 0.021 0.024 0.026 0.028 0.030 0.030

0.06 0.75 0.019 0.028 0.033 0.036 0.039 0.042 0.044 0.0431.00 0.030 0.042 0.048 0.052 0.054 0.056 0.055 0.0511.50 0.048 0.062 0.069 0.071 0.072 0.071 0.068 0.0630.50 0.013 0.023 0.029 0.032 0.035 0.038 0.038 0.038

0.08 0.75 0.027 0.038 0.042 0.046 0.049 0.054 0.057 0.0571.00 0.038 0.050 0.047 0.061 0.063 0.068 0.072 0.074

*Grate shown in Figure 4.16.


From Table 4.16 the resulting spread in flow = 2.00m. From Table 4.17,2.00m of spread results in an inlet capacity of 0.040 m3/s. Therefore, thetotal flow intercepted = 2x0.040 = 0.080 m3/s. The carryover flow = 0.18-0.08 = 0.10 m3/s.

For roads where few restrictions to inlet location may exist (i.e., high-ways and arterial roads), these charts can be used to establish minimumspacing between inlets. This is done by controlling the catchment area foreach inlet. The area is simplified to a rectangular shape of width and lengthwhere the length represents the distance between inlets.

Under special circumstances it may be necessary to install twin or dou-ble inlets to increase the inlet capacity. For reasons of interference by traf-fic such installations are usually installed in series, parallel to the curb.Studies17 have shown that where such installations exist on a continousgrade, the increases in inlet capacity rarely exceed 50 percent of the singleinlet capacity.

CURB INLETS

GUTTER INLETS

(a) Undepressed (b) Depressed (c) Deflector inlet

(d) Undepressed (e) Depressed

(f) Combination InletGrate placed directly in frontof curb opening depressed

DownHillFlow

Curb SlottedDrain

(h) Slotted DrainRoad

TypicalCross SectionSlot-In Sag

Figure 4.17 Stormwater inlets

(g) Multiple InletUndepressed


( )

The capacity of storm water inlets at a sag in the roadway is typicallyexpressed by weir and orifice equations.17 Flow into the inlets initially op-erates as a weir having a crest length equal to the length of perimeter whichflow crosses. The inlet operates under these conditions to a depth of about100mm. The quantity intercepted is expressed by the following:

Q = 0.l54 LD 1.5

Where Q = rate of discharge into the grate opening (m3/s)L = perimeter length of the grate, disregarding bars and

neglecting the side against the curb (m)D = depth of water at the grate (m)

When the depth exceeds 0.12m the inlet begins to operate as an orificeand its discharge is expressed by the following:

Q = 0.154 AD 0.5

Where Q = rate of discharge into the grate opening (m3/s)A = clear opening of the grate (m2)D = depth of water ponding above the top of the grate (m)

The inlet capacity of an undepressed curb inlet may be expressed by theequation:

where Q = discharge into inlets (m3/s) l = length of opening (m)g = gravitational acceleration (m3/s)d = depth of flow in gutter (m)

or

This assumes a gutter of wedge shaped cross-section with a cross-sectionalstreet slope of 10-3 to 10-1 with

Qo = flow in the gutter (m3/s)i = transverse slopes = hydraulic gradient of guttern = coefficient of roughness of gutter

The inlet capacity for a slotted drain may be determined from Figure4.19. The advantages of carryover are shown in Figure 4.18. If carryover isto be permitted, assume a length (LA) such that LA /LR is less than 1.0 butgreater than 0.4. It is suggested that L be in increments of 1.5m or 3m tofacilitate fabrication, construction and inspection. Pipe diameter is usuallynot a factor but it is recommended that an 500mm minimum be used. Itshould be carefully noted that, generally, the economics favor slotted drainpipe inlets designed with carryover rather than for total flow interception.Make certain that there is a feasible location to which the carryover maybe directed.

Determine the amount of carryover (C.O.) from Figure 4.18.At on-grade inlets where carryover is not to be permitted, LA must be at

least the length of LR.

Q/l = 1.47 x 10-3 d g/d

Q/l = 0.25 i0.579

s/n

Qo0.563


1.0

0.9

0.8

0.7

0.6

0.5 0.6 0.7 0.8 0.9 1.0Relative Length - (LA / LR)

Eff

icie

ncy

- (Q

a / Q

d)

Example: if 20% carryover (Qa / Qd = 80%) is allowed, then only 58% (LA / LR)of the total slotted drain length is required resulting in a 42% savings inmaterial and installation costs.

Figure 4.18 Slotted drain carryover efficiency

Figure 4.19 Slotted drain design Nomograph

20.0

10.0

9.08.0

7.0

6.0

5.0

4.0

3.0

484032

24

16

0.001

0.005

0.01

0.05

0.9

Long

itudi

nal S

lope

(m

/m)

-S

Tran

sver

se S

lope

Rec

ipro

cal -

Z

Turn

ing

Line

Leng

th (

m)

- L R

Dis

char

ge (

m3 /

S)

- Q

.20

.10

.09

.08

.07

.06

.05

.04

.03

.02


At sag inlets, the required length of slotted drain, LR, for total intercep-tion can be calculated from the following equation:

For sag inlets, LA should be at least 2.0 times the calculated LR to insureagainst the debris hazard. LA should never be less than 6m for sag inletcases.

The slot should be parallel to the curb and located in the gutter approxi-mately as shown.

DefinitionsS — Longitudinal gutter or channel slope, m/mSx — Transverse slope, m/mZ — Transverse slope reciprocal, m/md — Depth of flow, mL — Length of slot, mQ — Discharge, (m3/s)LR — Length of slot required for total interception, m (No carryover)LA — An assumed length of slot, mQd — Total discharge at an inlet, (m3/s)Qa — An asssumed discharge, (m3/s)

Slotted Drain is used effectively to intercept runoff from wide, flat areassuch as parking lots, highway medians — even tennis courts and airportloading ramps. In these installations, the drain is placed transverse to thedirection of flow, so that the open slot acts as a weir intercepting all of theflow uniformly along the entire length of the drain. The water is not col-lected and channeled against a berm, as required by a slot-on-grade instal-lation.

Slotted Drain has been tested for overland flow (sheet flow). These re-sults are published.18

The tests included flows up to 0.0011 m3/s per metre of slot. The testsystem was designed to supply at least 0.0007 m3/s per metre which corre-sponds to a rainstorm of 380mm per hour over a 20m wide roadway (6lanes). Slopes ranged from a longitudinal slope of 9 % and a Z of 16, to alongitudinal slope of 0.5% and a Z of 48. At the design discharge of 0.0007m3/s per metre, it was reported that the total flow fell through the slot as aweir flow without hitting the curb side of the slot. Even at the maximumdischarge of 0.0011 m3/s per metre and maximum slopes, nearly all theflow passed through the slot.

LR = 0.072 QD

h

90 mm


REFERENCES

14. Hare, C. M., Magnitude of Hydraulic Lossesat Junctions in Piped Drainage Systems, CivilEngineering Transactions, Institution of CivilEngineers, 1983, pp. 71-77.

15 . Howarth, D. A. and Saul, A. J, Energy LossCoefficients at Manholes, Proceedings 3rd In-ternational Conference on Urban StormDrainage, Goteburg, June 4-8, 1984, pp. 127-136.

16. Wright, K. K., Urban Storm Drainage Crite-ria Manual, Volume I, Wright-McLaughlinEngineers, Denver, Colorado, 1969.

17. Marsalek, J., Road and Bridge Deck Drain-age Systems, Ministry of Transportation andCommunications, Research and Develop-ment Branch, Ontario, Canada, Nov. 1982.

18. FHWA, Vol. 4, Hydraulic Characteristics ofSlotted Drain Inlets, Feb. 1980, Report No.FHWA-RD-79-106, Federal Highway Ad-ministration.

BIBLIOGRAPHYHandbook of Steel Drainage and Highway Con-struction Products, American Iron and Steel Insti-tute, 1983.73-3 Implementation Package for Slotted CMPSurface Drains, U.S. Dept. of Transportation, July1973.Jones, C. W., Design of Culverts.Bauer, W. J., Determination of Manning’s n for14 ft. Corrugated Steel Pipe, April 1969, BauerEngineering, Inc., Chicago, IL, 27 pp.Debris Control Structures, Hydraulic EngineeringCircular No. 9, Feb. 1964, Federal Highway Ad-ministration, U.S. Government Printing Office,Washington, D.C. 20402, 37 pp.Design Charts for Open Channel Flow, HydraulicDesign Series No. 3, 1961, U.S. Bureau of PublicRoads.Harrison, L. S., Morris, J. C., Normann, J. M., andJohnson, F. L., Hydraulic Design of Improved In-lets for Culverts, Hydraulic Engineering CircularNo. 13, Aug. I972, Federal Highway Administra-tion, Hydraulics Branch, HNG-31, Washington,D.C. 20590.Silberman, E., Effects of Helix Angle on Flow inCorrugated Pipes, Journal of the Hydraulics Divi-sion, American Society of Civil Engineers, Vol.96, Nov. 1970, pp. 2253-2263.Normann, J. M., Hydraulic Design of Large Struc-tural Plate Corrugated Metal Culverts, Unpub-lished Report, Jan. 1974, Hydraulics Branch,Bridge Division, Office of Engineering, FederalHighway Administration, Washington, D.C.20590, 17 pp.

1. Davis, C. B., Sorenson, K. E., Handbook ofApplied Hydraulics, 3rd Edition, 1969.

2. FHWA, Hydraulic Design of Highway Cul-verts, Hydraulic Design Series No. 5, ReportNo. FHWA-IP-85-15, Sept. 1985, FederalHighway Administration.

3. Design and Construction of Sanitary andStorm Sewers, Water and Pollution ControlFederation Manual of Practice No. 9 andAmerican Society of Civil Engineers Manu-als and Reports on Engineering Practice No.37, 1969.

4. Silberman, E., Dahlin, W.Q., Further Studiesof Friction Factors for Corrugated AluminumPipes Flowing Full, Project Report No. 121,April 1971, University of Minnesota, St.Anthony Falls Hydraulic Laboratory,Minneapolis, MN.

5. Grace, J. L., Jr., Friction Factors for Hydrau-lic Design of Corrugated Metal Pipe, Dept.of Defense, U.S. Corps of Engineers, Pro-ceedings of the Highway Research Board,U.S. Waterways Experimental Station, Vol.44, 1965.

6. Webster, M. J. and Metcalf, L. R., FrictionFactors in Corrugated Metal Pipe, Journal ofthe Hydraulic Division, American Society ofCivil Engineers, Vol. 85, Sept. 1959, pp. 35-67.

7. Brater, E. F., King, H.W., Handbook of Hy-draulics, 6th Edition, McGraw-Hill BookCompany, 1976.

8. Chow, V. T., Open Channel Hydraulics,McGraw-Hill Book Company, 1959.

9. Marsalek, J., Head Losses at Selected SewerManholes, Environmental Hydraulics Sec-tion, Hydraulics Division, National Water Re-search Institute, Canada Centre for InlandWaters, July 1985.

10. Ackers, P., An Investigation of Head Lossesat Sewer Manholes, Civil Engineering andPublic Works Review, Vol. 54, No. 637, 1959pp. 882-884 and 1033-1036.

11. Archer, B., Bettes, F. and Colyer, P. J., HeadLosses and Air Entrainment at SurchargedManholes, Report No. IT185, Hydraulics Re-search Station, Wallingford, 1978.

12. Black, R. G., Piggott, T. L., Head Losses atTwo Pipe Stormwater Junction Chambers,Proceedings Second National Conference onLocal Government Engineering, Brisbane,September 19-22, 1983, pp. 219-223.

13. deGrout, C. F., Boyd, M. J., ExperimentalDetermination of Head Losses in StormwaterSystems, Proceedings Second National Con-ference on Local Government Engineering,Brisbane, September 19-22, 1983.

88 modern sewer design

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